1. Field of the Invention
The invention relates in general to an OFDM (orthogonal frequency division multiplexing) communication system, and more particularly to symbol boundary detection device and method for use in an OFDM system.
2. Description of the Related Art
Recently, a multi-carrier (or multi-channel, multi-tone) communication technology has been widely used in data transmission, such as the OFDM (orthogonal frequency division multiplexing) technology for use in the IEEE 802.11a/g WLAN (wireless local area network). FIG. 1 is a block diagram showing a typical OFDM system. The OFDM system 100 respectively puts the to-be-transmitted data into N sub-channels in the frequency domain through a signal mapping device 101 in a transmitter. Then, an inverse fast Fourier transform (IFFT) device 102 transforms the to-be-transmitted data into a time-domain signal, to which a guard interval (GI) is added. Next, the time-domain signal with the guard interval passes through a parallel-to-serial converter (P/S) 104 and a DAC (digital-to-analog converter) 105, and is then transmitted in the channel 106. An ADC (analog-to-digital converter) 107 of the receiver samples the signal and removes the GI therefrom. Then, the signal passes through the serial-to-parallel converter (S/P) 110 and is fed to the fast Fourier transform (FFT) device 111, which transforms the signal back to the frequency domain signal. Then, channel compensations are respectively performed in the sub-channels, and finally a signal demapping device 113 demaps to signal into the originally transmitted data.
An output value of a set of N-point IFFT is referred to as a symbol. Because the channel impulse response (CIR) is not ideal in practical, a symbol passing through the channel 106 influences the subsequent symbol receiving operation in the receiver, thereby causing inter-symbol interference (ISI). In order to avoid this problem, a guard interval (GI) is added between the symbols. In general, the GI is added in a cyclic prefix (CP) manner. That is, the post-stage signal of the output symbol is duplicated to the front stage to serve as the GI. Consequently, it is possible to prevent the ISI from occurring and the N sub-channels from interfering with one another or each other when the CIR length does not exceed the GI. The part of signal processing is accomplished by adding a guard interval circuit 103 and removing a guard interval circuit 109 in FIG. 1.
However, the receiver has to determine the correct start position (i.e., the symbol boundary) of the time domain sampling signal inputted to the FFT device 111, that is, the proper time to perform FFT on the receiving sampling signal, before the GI is removed so as to avoid the ISI effectively. Thus, it is an important subject to perform a proper symbol boundary detection.
In the OFDM system 100, a known short preamble with the time domain periodicity for the synchronous processing of the time domain is firstly transferred in the packet or code frame. Then, known pilot symbols (or referred to as a long preamble) with the frequency domain signal for the channel estimation in the frequency domain are transferred so that channel compensation in the frequency domain is performed in the subsequent data symbols. A guard interval (referred to as GI2) is added before the long preamble time-domain signal, and the guard interval (referred to as GI) is also added before each data symbol in order to avoid the ISI. FIG. 2 shows the code frame architecture of a typical OFDM signal. As shown in FIG. 2, the short preamble is typically composed of some time-domain cyclic symbols with the auto-correlation property, and the long preamble symbols and subsequent data symbols are respectively added thereafter.
In the conventional boundary symbol detecting method, the receiver usually determines the proper start points of the long preamble symbols and the subsequent data symbols in order to remove the GI and to serve as the start basis for the subsequent input FFT according to the cyclic and auto-correlation properties of the short preamble. This method may be typically viewed as two parts.
First, the sampling signal of the received code frame is sent to the sliding delay correlator, and its output result is observed. The operation method of this sliding delay correlator is as follows:
                              c          k                =                              ∑                          n              =              0                                      N              -              1                                ⁢                                    r                              k                -                n                                      ·                                          r                                  k                  -                  n                  -                  N                                *                            .                                                          (                  1          -          1                )            
This operation correlates the sampling interval of the N points with the sampling interval of the previous N points, wherein the sampling interval of N points is sliding updated when the new sampling points are received. In Equation (1-1), rk denotes the k-th sampling value of the received code frame, N denotes the sampling number within one period of the short preamble, and ck is the k-th output value of the sliding delay correlator. According to the operation property of the sliding delay correlator, when the received signal is the cyclic signal, the absolute value of its output reaches a maximum. Accordingly, the absolute value of the output of the sliding delay correlator reaches the maximum when the short preamble is received, and reaches a relatively smaller value at other time. Thus, whether or not the short preamble has been received may be detected by comparing the absolute output value to a threshold. When the absolute output value ascends, from the relatively smaller value, to a value greater than this threshold, it means that the code frame has been detected (i.e., the receiving of the short preamble is started). Then, when the absolute output value descends to a value smaller than this threshold, it means that the guard interval of the long preamble symbols has been detected (i.e., the receiving of the short preamble is ended). FIG. 3 shows a schematic illustration of this mechanism.
Secondly, the sampling signal of the received code frame is transferred to a short preamble matched filter. The matched filter performs a plurality of linearly auto-regressive operations with respect to the sampling signal using the known short preamble as the coefficients. The operation method thereof is as follows:
                                          z            k                    =                                    ∑                              n                =                0                                            N                -                1                                      ⁢                                          r                                  k                  -                  n                                            ·                              p                n                *                                                    ,                            (                  1          -          2                )            wherein rk is the value of the k-th sampling points of the received code frame, pn is the known short preamble, N is the sampling number within one period of the short preamble, and zk is the k-th output result of the matched filter. Because the short preamble typically has some auto-correlation property, the output of the matched filter presents the estimated result of the time domain channel impulse response (CIR). In addition, because of the periodicity of the short preamble itself, if the output result of the matched filter is observed using a window having the length of N points, the estimated value of the CIR also presents in the window periodically. FIG. 4 shows a schematic illustration of this mechanism.
In the conventional method, there are two drawbacks. First, when determining the symbol boundary reference point, the maximum peak value in the observed window is detected, and then an early range is set forward according to this maximum peak value in order to cover the pre-cursor response of the CIR. Herein, the early range correlates with the boundary of the observed window. It is assumed that the start point of the early range is x sampling points in front of the maximum peak value, and the maximum peak value is distant from the start boundary of the window by y sampling points. If x<=y, then the start point of the early range is (y-x) point after the start boundary of the window, wherein the early range is from the start point, i.e. the (y-x)th point of the window, to the maximum peak value, i.e. the yth point of the window, as shown in FIG. 5A. If x>y, then the start point of the early range is (x-y) points before the end boundary of the window, wherein the early range ranges from the start point to the end boundary of the window as well as from the start boundary of the window to the maximum peak value, as shown in FIG. 5B. In the early range, the receiver further selects a position away from the maximum peak value by the fixed number of sampling points to serve as the symbol boundary reference point. This fixed method, however, lacks of flexibility, and misjudgment of the reference point will be made when the pre-cursor response is longer or has many relatively high peak values, thereby influencing the efficiency of the receiver.
Secondly, in the channel environment of the larger delay spread, the appearance of the guard interval of the long preamble symbols is often incorrectly estimated owing to the delay, and thus the timing for the long preamble symbols and the subsequent data symbols to enter the FFT device 111 is determined incorrectly.